This invention relates to optical transmission systems and, more particularly, to a method of and apparatus for providing an automatic level control circuit for nodes of such optical systems.
In optical communications systems, it is critical to maintain optical signal levels at their correct power settings. In particular in wavelength division multiplexed (WDM) systems, when a number of optical signals are transmitted and received at different wavelengths, many impairments can arise causing some channels to have a power level that is too high or low. In the case of long distance optical systems in which optical signals generate nonlinear effects such as self-phase or cross-phase modulation, it is desirable to ensure that the power levels in a single optical fiber span of the system remains well-equalized. In such a case, it would be desirable to place an adaptive equalizer in each amplifier node of the system. The time frame at which impairments arise as a result of unequal optical signals levels are extremely varied, from very slow changes occurring over a time span greater than 10 years in the case of component aging, to very fast (microsecond) transients occurring due to WDM add/drop or other protection switching operations.
In general, all of these problems will continue to get worse as more optical communication bandwidth is added to WDM systems, and new ultrabroadband amplifiers and components are developed. Clearly for each WDM system, it will be necessary to carefully optimize the placement of equalizer nodes so as to minimize the collective impairments due to incorrect power levels, crosstalk, self-phase and cross-phase modulation, and receiver penalties.
In each amplifier node of the WDM system it is desirable to maintain the transmitted power within a small range (typically about 1 dB variation). To accomplish power management at a node, it is possible to use an automatic level controller consisting of a power tap into a detector and an adjustable loss element (optical attenuator) with a simple feedback control loop which continuously compares the power level to a desired value, and adjusts the attenuation to maintain the power level at the desired value. However, real WDM networks include cascades of (typically nonlinear) fiber, amplifiers, and loss elements. The resulting system is nonlinear and potentially chaotic, such that when the simple control algorithm described is independently utilized in each span, the result can generate oscillations in power levels which are much worse (larger in amplitude and faster in frequency) than the original level fluctuations which the system is intended to correct. This type of network behavior has been observed by others (e.g., see the article by S. J. B. Yoo et al, xe2x80x9cObservation of Prolonged Power Transients in a Reconfigurable Multiwavelength Network and Their Suppression by Gain Clamping of Optical Amplifiersxe2x80x9d IEEE Photonics Technology Letters, V1O(11), p1659-1661; 1998). In their case, the power fluctuations were so severe that the use of active level stabilization was abandoned in favor of manual level tuning. In general, manual tuning will not be adequate for the increasingly complex networks being deployed. Further, while this kind of problem can occur in single wavelength networks, it is compounded by inter-wavelength interactions in WDM networks, and grows increasingly difficult to predict as the number of wavelengths increases. Finally, networks which deliberately add and drop wavelengths from a WDM span will need to be dynamically reconfigured.
Therefore what is desired is a simple power level control solution which allows an optical network to perform it""s own level stabilization, yet which prevents destructive feedback oscillations.
In accordance with the method and apparatus of the present invention, we have developed a way of eliminating feedback instabilities arising from interactions between automatic control loops at the amplifier nodes of an optical network by restricting operation to a single amplifier node at a time. Amplifier node activation is accomplished using a global control signal which is sequentially passed from an upstream node through all of the nodes of the system. By controlling the state of the global control signal, an upstream node has operational priority over downstream nodes.
More particularly, in accordance with our invention, an optical control apparatus comprises a control signal monitor and an adjustable optical transmission unit. The control signal monitor is responsive to a detected first state of an input global control signal for outputting a output global control signal at a first state, and is responsive to a detected second state of the input global control signal for outputting the output global control signal at the first state and for generating an enable signal. The adjustable optical transmission unit is responsive to the enable signal for controlling the output signal level of a received input optical signal. The adjustable optical transmission unit is (1) responsive to the enable signal for adjusting the output signal level to a predetermined level and for generating the okay signal when the output signal level adjustment is completed, and (2) responsive to the absence of said enable signal for maintaining the output signal level at its existing level. The control signal monitor is responsive to the okay signal for outputting the output global control signal at a second state.
In accordance with a system aspect of our invention, an optical communication system includes a plurality of optical links with each link including an optical fiber segment and an amplification node and with at least one of the amplification nodes including our above-described signal level control apparatus. Another aspect enables a global controller to receive system node status and to selectively address each system node one at a time to enable power (signal) level adjustments thereat.
According to other aspects of our invention, the adjustable optical transmission unit (1) includes one or more adjustable signal level elements selected from a group including an preamplifier, an amplifier, an attenuator, a filter, and an equalizer and (2) provides one or more types of output signal level adjustments selected from a group including a power level, a signal tilt level, and a level of one or more wavelengths of the output signal.